Connecting Sch 40 PCV to outside SDR-35 sewer line

I've remodeled my house and moved the original bathroom… my plan was to bring the "new" waste line outside thru the foundation and tie into the existing sewer line. The sewer line runs all the way to the back of the house where the original main stack was (only draining the kitchen waste and a basement washing machine into that line).

The new waste stack is 3" (shower, bathtub, lav and toilet). I wanted to bring that 3" line thru a combo and out the foundation to tie into the 6" sewer line. What's the best way to connect the 3" PVC sch 40 pipe thru the foundation to the SDR-35 pipe (6"). I was looking for a SDR-35 wye fitting with a sch 40 inlet… but haven't found anything.

I was also thinking that a typical hub'd fitting would be impossible to install (without digging the entire sewer line out) because you cannot separate the pipes (SDR-35) to get the fitting in-between them.

Probably not since the days of vitrified clay pipe. 6" would be "oversized" and modern codes try to limit oversizing drain lines because of flow velocity issues. When I have seen it, it was the city's clay branch to the property line and the house pipe was ALWAYS reduced to 3" ro 4".

You do NOT insert the fitting by "flexing" the line. You cut a section out and insert the fitting (which will have short pipe stubs on the ends), into the gap using "repair couplings', also known as Fernco or transition couplings. With sch 40 x SDR transition couplings, the fitting will have sch. 40 stubs on each end.

Odd, because I have never seen a residential septic tank with anything other than a 4" inlet.

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There is NO septic system anymore... well the tank might be (buried somewhere), but I'm paying for sewage and I happened to find this SDR pipe buried next to the house.

Like I said, the main stack is at the back of the house and there is a 4" CI pipe coming thru the foundation. I "assume" it once lead straight back (backyard) into a septic tank.

Fast-forward a few years and the city put in a sewage system and the house was tied into it. How they did it I don't know. But the connection is outside somewhere as the 4" CI is still in the foundation at the back of the house. I imagine that they cut the CI pipe running to the septic tank, trenched in the SDR pipe from the street (all the way to the back of the house) and joined to the CI. I accidently discovered the SDR pipe when I was digging for a new bilco door.

I can't say 100% that it is, but given the property layout, proximity to the house and current location of waste line thru the foundation...

Say it is the main drain line... is there an issue using the fernco saddle tap? It seems much easier, quicker and cheaper (not that I'm pinching pennies) to go that route. Then to buy some 6" PVC, fitting, 2 rubber couplers, etc...

Probably not since the days of vitrified clay pipe. 6" would be "oversized" and modern codes try to limit oversizing drain lines because of flow velocity issues. When I have seen it, it was the city's clay branch to the property line and the house pipe was ALWAYS reduced to 3" ro 4".

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I went looking for such a code requiring 6 inches and did not find it. ( Surprisingly I did find the requirement for some residential storm sewers that hooked to the street storm sewer)

When some friends had overhead sewers put in, IIRC the plumber had to put in 6 inch sewers from the side of the house and across the front joining the existing sewer. It then joined to the existing 6-inch sewer on the house side of the sidewalk. I am now suspecting that maybe the option might have been to use 4 inch by code, but that would have needed a larger slope. There may not have been enough drop to provide a 1/4 inch per foot drop, so they may have been able to use a shallower 1/8 inch slope with 6-inch SDR pipe given the fairly deep frost line. Alternatively, a shallower slope would have required slightly less digging. This digging was done by hand to minimize lawn damage and to separate existing sod from top soil (not 100% sure of that aspect) and separating good top soil from clay (I am 100% sure of that aspect) . They even dug under a sidewalk in the middle of the front yard to pass the sewer pipe. When putting the lower soil layers back, they pounded with sledgehammers! That whole process was amazingly effective at keeping the lawn much nicer than you expect from sewer trenching.

quote; . There may not have been enough drop to provide a 1/4 inch per foot drop, so they may have been able to use a shallower 1/8 inch slope with 6-inch SDR pipe given the fairly deep frost line.

BOTH 4" and 6" can be installed with 1/8" pitch so that is not a viable reason for using 6". In fact, if a precise interpretation of the code were done, 6" would be REJECTED because its velocity, and thus its scouring action would be compromised, if 4" were adequate. Which would ALWAYS be the case for a residence.